Neutron detectors are used to detect and discriminate neutrons in ambient gamma and beta radiation fields. Such applications include monitoring neutrons in and around nuclear reactors for reactor control, reactor safety, reactor monitoring, nuclear material (fuel) accountancy, and radiological protection. Neutron detectors can also be used in security applications for detection of contraband fissionable materials, for industrial applications such as neutron radiography and tomography, for scientific research including neutron scattering and particle accelerator experiments in nuclear physics and material science investigations, and for detection of exotic particles such as neutrinos and dark matter.
There are currently several types of media for detecting neutrons. These include media which use Helium-3 gas, boron coated particles, boron trifluoride gas, lithium coatings or foils, solid or glass scintillators loaded with a neutron-reactive element (such as lithium, boron or gadolinium), and boron-loaded liquid scintillators.
Helium-3 filled ionization chamber tubes have been extensively used in the field because of their good neutron-to-gamma discrimination capability. However, there is currently a global shortage of Helium-3. As a result, it is necessary to design new neutron detectors relying on other mechanisms, and that are as effective as He-3 detectors.
The boron-10 capture process has been suggested. With a cross section of 3838 barns, an incident thermal neutron may be captured by boron-10 and produces Lithium-7 and alpha as follows [1]:
                             0        1            ⁢      n        +                           5        10            ⁢      B        →      {                                                                                                           3                  7                                ⁢                Li                            ⁡                              (                                  1.015                  ⁢                                                                          ⁢                  MeV                                )                                      +                                                                               2                  4                                ⁢                He                            ⁡                              (                                  1.777                  ⁢                                                                          ⁢                  MeV                                )                                                                          6            ⁢            %                                                                                                                               3                  7                                ⁢                Li                            ⁡                              (                                  0.840                  ⁢                                                                          ⁢                  MeV                                )                                      +                                                                               2                  4                                ⁢                He                            ⁡                              (                                  1.47                  ⁢                                                                          ⁢                  MeV                                )                                      +                          γ              ⁡                              (                                  0.478                  ⁢                                                                          ⁢                  MeV                                )                                                                          94            ⁢            %                              Lithium-7 and alpha particle have a short range and will deposit their energies into the surrounding medium. If the environment is a scintillator, a considerable number of optical photons can be emitted following this interaction. The optical photons are easily transformed to an electrical pulse by a photomultiplier for processing and achieving an efficient neutron detector. Therefore, this interaction is monitored to count neutrons.
U.S. Pat. No. 3,372,127 to Thomas et al. describes several boron-loaded liquid scintillator compositions. These compositions comprise enriched trimethyl borate (0.95 B10) and isopropyl biphenyl with varying amounts of either naphthalene or 1,4-di-[2-(5-phenyloxazolyl)]-benzene, and either 2-phenyl-5-(4-biphenylyl)-1,3,4-oxadiazole, 2-(1-naphthyl)-5-phenyloxazole or 9,10-diphenyl anthracene. However, scintillator compositions such as those described by Thomas et al. have several drawbacks. For instance, they typically contain a high content of trimethyl borate, which is unstable when exposed to moisture, is flammable, and yields a low light output or signal. In addition, many liquid scintillators, and particularly boron loaded liquid scintillators, use toxic and flammable liquid scintillation solvents, which are difficult to handle and incorporate into detectors. Furthermore, because the light output of these liquid scintillators is low, it is more difficult to discriminate neutron signal from noise and gamma-beta background radiation.
Accordingly, there is a need for new and improved liquid scintillators which are effective for neutron detection.